ladybird vs ladybug

ladybird and ladybug both are nouns.

Word	Noun	Adjective	Verb	Adverb
ladybird	Yes	No	No	No
ladybug	Yes	No	No	No

As nouns, ladybird and ladybug are synonyms defined as:

ladybird and ladybug: small round bright-colored and spotted beetle that usually feeds on aphids and other insect pests

Other synonyms of ladybird include lady beetle, ladybeetle, ladybird beetle.

ladybird (noun)	ladybug (noun)
small round bright-colored and spotted beetle that usually feeds on aphids and other insect pests	small round bright-colored and spotted beetle that usually feeds on aphids and other insect pests